Conventional collecting and condensing designs for electromagnetic radiation emphasize collecting and redirecting the maximum amount of light from a single point source radiating isotropically. In doing so, the ability of these designs to concentrate radiation flux into a small spot size is compromised. Adapting these designs to produce a small spot size results in a decrease in radiation flux because the emphasis of conventional designs (i.e., the collection and redirection of the maximum amount of light) conflicts with the goal of concentrating the light flux into the smallest possible spot size when the light originates from conventional non-coherent light sources. Thus, small spot size images may be obtained only with the corresponding penalty of decreased flux density.
There are two basic designs for optical collecting and condensing systems that are in common use. The first is a system of condenser lenses such as illustrated in FIG. 1. Condenser lenses have several problems, including the creation of chromatic and spherical aberrations, the high cost of corrective optics, the inherent difficulty in aligning the lenses and the large amount of space such a system demands. Ellipsoidal reflectors as shown in FIG. 2 are also used in prior art systems. Their problems include high cost and the unavoidable magnification of the image which reduces the flux density at the image. Both of these systems (FIGS. 1 and 2) tend to emphasize the collection and redirection of the maximum amount of light from a single point source as discussed above. Thus, they fail to optimize both spot size and light density.
A variation to the system illustrated in FIG. 1 was previously described in French Patent #1383413. In this configuration a spherical concave mirror having a center of curvature and an optical axis is used to collect and condense light from a filament source into a light guide. The source is placed at the center of curvature of the mirror and light is focused into the light guide at a point opposite the side on which the spherical mirror is located. Enhanced performance is achieved by placing a second spherical mirror on the side opposite from the primary spherical reflector to focus light back through the source to the primary reflector. A hole placed in the center of the secondary reflector allows placement of a light guide along the optical axis to collect the reflected radiation. Also described in Patent #1383413 is the use of a primary elliptical mirror configured as shown in FIG. 2, except that a secondary spherical reflector, having a center of curvature coincident with the source and placed at a distance between the primary and secondary focal points of the elliptical reflector, is used to increase the amount of collected light into a light guide. The light guide is placed along the optical axis at the secondary focal point and a hole is placed in the secondary reflector to permit light to enter the light guide.
U.S. Pat. No. 4,757,431, the disclosure of which is incorporated herein by reference, describes an improved condensing and collecting system employing an off-axis spherical reflector to increase the flux density at the target. As shown in FIG. 3, the prior art off-axis system has a source transversely displaced from the optical axis of the reflector and a target placed at an approximately symmetrical position with respect to the optical axis. However, such a system has certain disadvantages arising from the "off-axis displacement" of the source and target including the presence of astigmatism parallel to the direction of the off-axis displacement and the physical limitations inherent in the requirement to minimize this off-axis distance. The effect of astigmatism is to decrease the concentrating efficiency of the system and thereby reduce the flux collected at the target. Also, the requirement to minimize the off-axis distance between the source and the target due to the resulting astigmatic distortion imposes limitations on the physical dimensions of the source and target in such a system.
Accordingly, it is an object of the present invention to provide an on-axis optical system which enhances the collection of light emitted from a localized source of electromagnetic radiation and subsequently acquired by a small optical target.
It is another object of the present invention to provide an on-axis optical system comprising a source, a primary reflector and an optical target, in which both the source and optical target are in line with the optical axis of the primary reflector, but axially offset from one another.
It is a further object of the present invention to provide an on-axis optical system which eliminates the astigmatic aberrations and physical limitations inherent in an off-axis system.